The present invention relates to emulsifier-free finely dispersed systems of the water-in-oil type, preferably as cosmetic or dermatological preparations.
Emulsions are generally taken to mean heterogeneous systems which consist of two liquids which are immiscible or have only limited miscibility with one another, which are usually referred to as phases. In an emulsion, one of the two liquids is dispersed in the form of very fine droplets in the other liquid.
If the two liquids are water and oil and if oil droplets are finely dispersed in water, then this is an oil-in-water emulsion (O/W emulsion, e.g. milk). The basic character of a O/W emulsion is defined by the water. In a water-in-oil emulsion (W/O emulsion, e.g. butter), the principle is reversed, the base character here being determined by the oil.
In order to achieve permanent dispersion of one liquid in another, emulsions in the traditional sense require the addition of an interface-active substance (emulsifier). Emulsifiers have an amphiphilic molecular structure, consisting of a polar (hydrophilic) and a nonpolar (lipophilic) molecular moiety, which are spatially separate from one another. In simple emulsions, finely dispersed droplets of one phase, surrounded by an emulsifier shell, (water droplets in W/O emulsions or lipid vesicles in O/W emulsions) are present in the second phase. Emulsifiers lower the interfacial tension between the phases by positioning themselves at the interface between the two liquids. At the phase boundary, they form oil/water interfacial films, which prevent irreversible coalescence of the droplets. Emulsions are frequently stabilized using emulsifier mixtures.
Traditional emulsifiers can, depending on their hydrophilic molecular moiety, be divided into ionic (anionic, cationic and amphoteric) and nonionic: the most well known example of an anionic emulsifier is soap, which is usually the term used for the water-soluble sodium salts or potassium salts of saturated or unsaturated higher fatty acids.
Important examples of cationic emulsifiers are quaternary ammonium compounds.
The hydrophilic molecular moiety of nonionic emulsifiers frequently consists of glycerol, polyglycerol, sorbitans, carbohydrates and polyoxyethylene glycols, and, in most cases, is linked to the lipophilic molecular moiety via ester and ether bonds. The lipophilic molecular moiety usually consists of fatty alcohols, fatty acids or isofatty acids. By varying the structure and the size of the polar and nonpolar molecular moiety, the lipophilicity and hydrophilicity of the emulsifiers can be varied within wide limits.
A decisive factor for the stability of an emulsion is the correct choice of emulsifiers. The characteristics of all substances present in the system are to be taken into consideration. In the case of, for example, skin care emulsions, polar oil components and, for example, UV filters lead to instability. As well as the emulsifiers, therefore, other stabilizers are also used which, for example, increase the viscosity of the emulsion and/or act as a protective colloid.
Emulsions are an important type of product in the field of cosmetic and/or dermatological preparations.
Cosmetic preparations are essentially used for skin care. The main aim of skin care in the cosmetics sense is to strengthen or rebuild the skin""s natural function as a barrier against environmental influences (e.g. dirt, chemicals, microorganisms) and against the loss of endogenous substances (e.g. water, natural fats, electrolytes). If this function becomes impaired, increased resorption of toxic or allergenic substances or infection by microorganisms may result, leading to toxic or allergic skin reactions.
Another aim of skin care is to compensate for the loss by the skin of grease and water caused by daily washing. This is particularly important if the natural regeneration ability is inadequate. Furthermore, skin care products should protect against environmental influences, in particular against sun and wind, and delay skin ageing.
Cosmetic preparations are also used as deodorants. Such formulations are used to control body odour which is produced when fresh sweat, which is in itself odourless, is decomposed by microorganisms.
Medicinal topical compositions usually comprise one or more medicaments in an effective concentration. For the sake of simplicity, in order to distinguish clearly between cosmetic and medicinal use and corresponding products, reference is made to the legal provisions in the Federal Republic of Germany (e.g. Cosmetics Regulation, Foods and Drugs Act).
The use of customary emulsifiers in cosmetic or dermatological preparations is in itself acceptable. Nevertheless, emulsifiers, like ultimately any chemical substance, may in certain circumstances cause allergic reactions or reactions based on oversensitivity of the user.
For example, it is known that certain light dermatoses are triggered by certain emulsifiers, but also by a variety of fats and simultaneous exposure to sunlight. Such light dermatoses are also called xe2x80x9cMallorca acnexe2x80x9d. There has thus been no lack of attempts to reduce the amount of customary emulsifiers to a minimum, in the ideal case even to zero.
A reduction in the required amount of emulsifier can, for example, be achieved by taking advantage of the fact that very finely divided solid particles have an additional stabilizing action. The solid substance accumulates at the oil/water phase boundary in the form of a layer, as a result of which coalescence of the dispersed phases is prevented. It is not the chemical properties of the solid particles which are of fundamental importance here, but the surface properties.
Around 1910, Pickering prepared paraffin/water emulsions which were stabilized merely by the addition of various solids, such as basic copper sulphate, basic iron sulphate or other metal sulphates. This type of emulsion is thus also referred to as a Pickering emulsion. For this type of emulsion, Pickering postulated the following conditions:
(1) The solid particles are only suitable for stabilization if they are significantly smaller than the droplets of the inner phase and do not have a tendency to form agglomerates.
(2) An important property of an emulsion-stabilizing solid is also its wettability. I.e. in order to stabilize an O/W emulsion, the solid has, for example, to be more readily wettable by water than by oil.
The original forms of Pickering emulsions initially surfaced, as it were, as undesired secondary effects in a variety of industrial processes, such as, for example, in secondary oil recovery, the extraction of bitumen from tar sand and other separation processes involving two immiscible liquids and fine, dispersed solid particles. These are generally W/O emulsions which are stabilized by mineral solids. Accordingly, investigation of corresponding systems, such as, for example, the oil/water/soot or oil/water/slate dust systems was initially the focus of research activity.
Basic experiments have shown that one characteristic of a Pickering emulsion is that the solid particles are arranged at the interface between the two liquid phases where they form, as it were, a mechanical barrier against the mixing of the liquid droplets.
It is a relatively new technical development to use Pickering emulsions as a base for cosmetic or dermatological preparations.
One way of achieving solids stabilization in a cosmetic or dermatological preparation is, according to May-Alert (Pharmazie in unserer Zeit [Pharmacy in our time], Vol. 15, 1986, No. 1, 1-7) for example, to use emulsifier mixtures which contain both anionic and cationic surfactants. Since mixing anionic and cationic surfactants produces precipitates of insoluble, electro-neutral compounds, deliberate precipitation of these neutral surfactants in the oil/water interface makes it possible to achieve additional solids stabilization in the sense of a Pickering emulsion.
EP-A-0 686 391 describes water-in-oil emulsions which are free from surface-active substances and are stabilized only by solids. Stabilization is achieved here using spherical polyalkylsilsesquioxane particles which have a diameter of from 100 nm up to 20 Am. According to the above, these emulsions can be referred to as Pickering emulsions.
In addition to the described Pickering emulsions, the prior art describes further emulsifier-free, finely dispersed cosmetic or dermatological preparations which are generally referred to as hydrodispersions. Hydrodispersions are dispersions of a liquid, semi-solid or solid internal (discontinuous) lipid phase in an outer aqueous (continuous) phase. Hydrodispersions, like other emulsions, are metastable systems and therefore have a tendency to convert into a state of two mutually coherent discrete phases.
In the case of hydrodispersions of a liquid lipid phase in an outer aqueous phase, stability can be ensured, for example, by constructing, in the aqueous phase, a gel structure in which the lipid droplets are stably suspended. DE-A 44 25 268 describes stable finely dispersed, emulsifier-free cosmetic or dermatological preparations of the oil-in-water type, which, in addition to one oil phase and one water phase, comprise one or more thickeners from the group consisting of acrylic polymers, polysaccharides and alkyl ethers thereof, where these thickeners must not cause any lowering of the interfacial tension.
Using hydrodispersions as a basis, DE-A 43 03 983 discloses cosmetic or dermatological light protection formulations which comprise inorganic micropigments as UV filter substances. The formulations consist of an inner lipid phase and an outer aqueous phase and are essentially free from emulsifiers. The inorganic micropigments are incorporated into the lipid phase of this hydrodispersion.
The object of the present invention was to extend the prior art to include cosmetic or dermatological preparations of the water-in-oil type in which it is not necessary to use any emulsifiers of a conventional type.
Surprisingly, this object is achieved by emulsifier-free cosmetic or dermatological preparations which are finely dispersed systems of the water-in-oil type, comprising
1. an oil phase,
2. an aqueous phase and
3. at least one type of micronized, inorganic pigments which
a) have an average particle size of less than 200 nm, and whose particles
b) have both hydrophilic and lipophilic properties, i.e. have an amphiphilic character, and thus position themselves at the water/oil interface, and which
c) are selected from the group consisting of metal oxides, which
d) are optionally coated on the surface
and also
optionally comprising further cosmetic or pharmaceutical auxiliaries, additives and/or active substances.
The amphiphilic character of the inorganic pigments according to the invention is evident, for example, from the fact that the latter are dispersible both in water and in oil.
The preparations according to the invention are mixtures of oils or oil-soluble substances and water or water-soluble components, which are stabilized by adding micronized solids particles and which do not contain an emulsifier in the traditional sense. One way of explaining the stability of these preparations is that the pigment particlesxe2x80x94as shown diagrammatically in FIG. 1xe2x80x94attach themselves to the droplets of the disperse phase and form, as it were, a mechanical barrier, which prevents coalescence of the droplets.
The preparations according to the invention are extremely satisfactory preparations in every respect, whose aqueous/fatty phase ratio can be varied within extraordinarily wide limits.
It is advantageous to choose the average particle diameter of the pigments used to be between 1 nm and 200 nm, particularly advantageously between 5 nm and 100 nm.
The water phase content of the formulations according to the invention is preferably chosen from the range of from 0.5 to 75% by weight, based on the total weight of the formulations.
It is essentially insignificant for the present invention in which of the potentially naturally occurring modifications the metal oxides are present.
It is advantageous for the purposes of the present invention to stabilize the Pickering emulsions using untreated, virtually pure pigment particles, in particular those which can also be used as dye in the food industry and/or as absorber of UV radiation in sunscreens. Examples of advantageous pigments are the zinc oxide pigments obtainable from Merck and those which are obtainable under the trade names zinc oxide neutral from Haarman and Reimer or NanoX from Harcros Chemical Group.